Introduction

With the exception of the craniofacial skeleton and the clavicle, bone formation during development occurs through a process called endochondral ossification, whereby cartilage is formed as a skeletal tissue, calcified and replaced by bone. Much of the craniofacial skeleton consists of ‘membrane’ bones that form as a result of intramembranous ossification and without a cartilaginous intermediate. The clavicle is the only ‘membrane’ bone in mammals outside the craniofacial skeleton. The axial and appendicular skeletons and portions of the cranial skeleton (calvaria, otic capsule) arise from mesoderm.

In the embryo, bone formation occurs following an orderly and carefully orchestrated differentiation of mesenchymal cells into chondroblasts, perichondrium, periosteum and osteoblasts (Hall, 1987) (Fig. 1.1). Then growth plates are established, first to lengthen bones and then, in the case of long bones, to shape the forming epiphyses.

As part of this process, there is a complex series of events that involves the formation of chondroblasts and then their maturation into chondrocytes. Only mature hypertrophic chondrocytes establish a calcified extracellular matrix, which is then partly resorbed through a process involving angiogenesis. This first occurs early in development within the diaphysis and then later in the growth plates (Fig. 1.1). The calcified cartilage then acts as a template on which osteoblasts form woven bone, which is eventually resorbed and replaced with a mature trabecular bone within the epiphyses and the diaphysis.